Previous multiple access schemes for satellite communications have operated on a variety of bases. One type is where resources are assigned by some control centre on a demand basis, such as Time Division Multiple Access (TDMA).
Other systems include variants of Code Division Multiple Access (CDMA), including Frequency Hopped Code Division Multiple Access (FH-CDMA) and Direct Sequence Code Division Multiple Access (DS-CDMA). Both these systems use spread spectrum transmission techniques. In FH-CDMA, user signatures occupy a time varying narrow band, while in DS-CDMA signatures occupy the full bandwidth.
In FH-CDMA, each user modulates a hopping carrier over the full transmission bandwidth. Special sequences are used to minimise hits between hopping patterns of different users. The number of carriers available is B/H, where B is the available bandwidth to be shared and H is the hopping rate. In FH-CDMA only one carrier frequency is transmitted per time interval.
In DS-CDMA a modulated carrier is multiplied by a high symbol rate frequency spreading signal, which is usually a pseudo random binary sequence. At the receiver the "despreading" operation is performed by multiplying the received signal with a locally generated version of the original spreading signal.
U.S. Pat. No. 3,292,179 to Magnuski discloses a system which utilises a fixed range of frequencies and time shifts to address users. It does not provide an interleaved frequency system in which address and data are comprised in a single transmission.
It is the object of the present invention to provide an improved multiple user communications system, particularly but not exclusively for satellite communications, which ameliorates at least some of the deficiencies of the prior art.